Modern high bypass turbofan engines incorporate various types of airfoils such as fan blades, nozzles and struts. In particular, wide chord fan blades improve aerodynamic efficiency and improve tolerance to impact from foreign objects. To minimize system weight, airfoils and their root attachments (dovetails) are often made hollow by joining premachined sections together into a final assembly. Common internal core constructions rely on the use of radial ribs or trusses evenly spaced within the cavity. These features serve as the support structures for the outer skins, and act to carry centrifugal loading, support the skins against gas pressure and foreign object impact loading, such as from birds, and provide stiffness.
Strength requirements in some components, particularly airfoils in the early stages of gas turbine engines, vary across the part. For instance, impact strength is a primary concern on the forward edge of the airfoil. Equally spaced rib or truss members provide uniform strength across the blade even though strength requirements are not uniform. Certain airfoil areas would benefit by more support. Adding more ribs or trusses or increasing skin thickness accomplishes this effect, but at a substantial weight penalty.
In a gas turbine engine, added weight in blades necessitates added weight in other hardware, including the disk, stationary structures, and the containment system. This in turn, increases fuel burn and customer cost. More ribs or trusses also add to machining time, tooling costs, and inspection time, which further drives up production costs.
Accordingly, the present invention provides a new arrangement of internal structural members that improves the impact strength of hollow airfoil components subjected to impact from foreign objects, such as birds, while maintaining existing aerodynamic function of the airfoil, minimizing airfoil weight and minimizing machining and inspection time.